Victor Forge
The pupil is an opening in the iris that lets light pass through to the retina. The iris sphincter muscle (or pupillary sphincter) and the iris dilator muscle (or pupillary dilator) are a pair of antagonistic muscles that control the diameter of the pupil. The iris sphincter muscle encircles the pupil and constricts it in bright light or during accommodation, while the iris dilator muscle dilates the pupil to allow more light to enter the eye.
| Characteristics | Values |
|---|---|
| Muscle controlling pupil constriction | Iris sphincter muscle (pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibres) |
| Muscle controlling pupil dilation | Iris dilator muscle (pupil dilator muscle, pupillary dilator, radial muscle of iris, radiating fibres) |
| Location of iris sphincter muscle | In the coloured part of the eye (iris) |
| Iris sphincter muscle fibres | Located near the pupillary margin and slightly anterior to the pigmented epithelium of the iris |
| Iris sphincter muscle function | Constricts the pupil in bright light or during accommodation |
| Iris dilator muscle fibres | Arranged radially from the sphincter to the ciliary border |
| Iris dilator muscle function | Dilates the pupil to allow more light to enter the eye |
| Control of iris sphincter muscle | Parasympathetic nervous system, Edinger-Westphal nucleus (EWN) |
| Control of iris dilator muscle | Sympathetic nervous system, superior cervical sympathetic ganglion |
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What You'll Learn
The iris sphincter muscle constricts the pupil
The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the coloured part of the eye, the iris. The iris sphincter muscle encircles the pupil and functions to constrict it in response to bright light or during accommodation. This process is known as the pupillary light reflex.
The iris sphincter muscle is under the control of the parasympathetic nervous system. Specifically, it receives parasympathetic innervation via the short ciliary nerves, which originate from the Edinger-Westphal nucleus of cranial nerve III. This innervation leads to the constriction of the pupil, a process known as miosis. The parasympathetic fibres that serve the sphincter muscle synapse in the ciliary ganglia and terminate on muscarinic receptors of the muscle fibres.
The function of the iris sphincter muscle is to control the amount of light that reaches the retina at the back of the eye. By constricting the pupil, the iris sphincter muscle reduces the amount of light entering the eye, maintaining clear vision. This is particularly important in bright light conditions, where a smaller pupil helps to prevent excessive light from reaching the retina.
The iris sphincter muscle is also involved in the accommodative reflex, an involuntary response that occurs when visual focus shifts from a distant object to one that is closer. During this reflex, the pupil aperture, lens shape, and convergence are altered. The sphincter pupillae narrows the pupil to prevent diverging light rays from creating a blurred image.
In summary, the iris sphincter muscle, or sphincter pupillae, constricts the pupil in response to bright light and during accommodation. This muscle is controlled by the parasympathetic nervous system and plays a crucial role in regulating the amount of light that reaches the retina, thus maintaining clear vision.
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The iris dilator muscle dilates the pupil
The iris is a part of the eye that regulates the amount of light that reaches the retina. The iris has an opening called the pupil, and its diameter is controlled by two opposing muscles: the sphincter pupillae and the dilator pupillae. The sphincter pupillae constricts the pupil, while the dilator pupillae, or iris dilator muscle, dilates it.
The iris dilator muscle is a smooth muscle of the eye, running radially in the iris. It consists of a spoke-like arrangement of modified contractile cells called myoepithelial cells. These cells are stimulated by the sympathetic nervous system. When these cells contract, the pupil dilates, allowing more light to enter the eye.
The dilator muscle is innervated by postganglionic sympathetic nerves arising from the superior cervical ganglion as the sympathetic root of the ciliary ganglion. They travel via the internal carotid artery through the carotid canal to the foramen lacerum. They then enter the middle cranial fossa above the foramen lacerum, passing through the cavernous sinus in the middle cranial fossa. They then travel with the ophthalmic artery in the optic canal or on the ophthalmic nerve through the superior orbital fissure. From there, they travel with the nasociliary nerve and then the long ciliary nerve. They then pierce the sclera, travelling between the sclera and choroid to reach the iris dilator muscle.
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The parasympathetic nervous system controls the sphincter muscle
The pupil is the aperture in the iris, which changes in size to adjust the amount of light entering the eye and reaching the retina. The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the coloured part of the eye, the iris. The muscle fibres are located near the pupillary margin and encircle the pupil of the iris.
The iris sphincter muscle is controlled by the parasympathetic nervous system. The parasympathetic fibres that serve the sphincter muscle originate from the Edinger-Westphal nucleus of cranial nerve III. The parasympathetic nervous system helps to control your body's response during times of rest. It carries signals that return your body's systems to their standard activity levels. The parasympathetic nervous system can constrict the pupil to limit how much light enters your eyes.
The iris sphincter muscle functions to constrict the pupil in bright light via the pupillary light reflex or during accommodation. By controlling the pupil's diameter, the iris controls the amount of light that reaches the retina. The accommodative reflex of the eye is an involuntary response when the visual focus switches from an object at a distance to one that is closer. The reflex is controlled by the parasympathetic nervous system and involves three actions: alteration of pupil aperture, lens shape, and convergence.
The sphincter pupillae causes the narrowing of the pupil to prevent diverging light rays from the corneal periphery from creating a blurred image. The iris sphincter muscle receives parasympathetic innervation via the short ciliary nerves, leading to pupillary constriction (miosis) and accommodation. The iris dilator muscle, on the other hand, is controlled by the sympathetic nervous system and causes dilation of the pupil (mydriasis).
The iris sphincter muscle plays a primary role in controlling rapid pupil dilation with the onset of movement. The parasympathetic nervous system controls the constriction of the pupil, while the sympathetic nervous system controls the dilation of the pupil.
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The sympathetic nervous system controls the dilator muscle
The pupil is the aperture at the centre of the iris, which changes in size to adjust the amount of light entering the eye and reaching the retina. The iris sphincter muscle and the iris dilator muscle control pupil size, with the former constricting and the latter dilating the pupil. The iris sphincter muscle is controlled by the parasympathetic nervous system, while the iris dilator muscle is controlled by the sympathetic nervous system.
The sympathetic nervous system (SNS) is one of the two divisions of the autonomic nervous system (ANS), the other being the parasympathetic nervous system (PNS). The SNS and PNS work unconsciously and in opposite ways to regulate many functions and parts of the body. The SNS is responsible for the "fight or flight" response, while the PNS controls the "rest and digest" response. The SNS innervates tissues in almost every organ system and regulates various bodily processes, including pupil diameter, gut motility, and urinary output.
The iris dilator muscle is a smooth muscle located in the outer part of the iris. It is primarily regulated by the SNS through adrenergic receptors, particularly the α1 receptors. The SNS is the major source of innervation for the iris dilator muscle, stimulating α1, α2, and β2 adrenergic receptors.
The pupillary dilation pathway is a sympathetically driven response that begins in the hypothalamus and ends with the contraction of the dilator pupillae muscle. Pupillary dilation may result from any physical or emotional stress that triggers the autonomic SNS, which is mediated by the hypothalamus. The pupillary dilation pathway is a three-neuron pathway, and disruptions anywhere along the oculosympathetic pathway may cause Horner syndrome, characterised by ptosis, miosis, and anhydrosis of the face due to loss of sympathetic innervation.
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The accommodative reflex
The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the coloured part of the eye called the iris. The sphincter muscle fibres are located near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris. It encircles the pupil of the iris and functions to constrict the pupil in bright light via the pupillary light reflex or during accommodation. By controlling the pupil's diameter, the iris controls the amount of light that reaches the back of the eye at the retina.
The iris dilator muscle fibres are radially oriented and lie along the base of the posterior epithelium. The dilator muscle is under sympathetic control in mammals. The sympathetic fibres course along the spinal cord to T1 to T3, leave in the ventral roots, course through the anterior mediastinum, and run anteriorly with the internal carotid to the cranial cervical ganglion, where they synapse. They then travel to the eye in association with the ocular vascular supply.
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Frequently asked questions
The iris sphincter muscle and the iris dilator muscle.
The iris sphincter muscle constricts the pupil. It is controlled by the parasympathetic nervous system.
The iris dilator muscle dilates the pupil. It is controlled by the sympathetic nervous system.
The pupil size is controlled by the iris sphincter and dilator muscles, which constrict and dilate the pupil, respectively. Pupil size can also be impacted by luminance, accommodation, and various brain states related to arousal, locomotion, emotion, attention, and cognitive load.
The pupil size changes through the contraction and relaxation of the iris sphincter and dilator muscles. The contraction of the circular iris sphincter muscle and the relaxation of the radial iris dilator muscle lead to constriction (miosis) and a smaller pupil size. Conversely, the contraction of the radial iris dilator muscle and the relaxation of the iris sphincter muscle lead to dilation (mydriasis) and a larger pupil size.
